1. Field of the Invention
This invention relates to apparatus and methods for immersing an object in a liquid.
2. Description of Related Art
A wafer fabrication process typically forms many identical integrated circuits upon each of several silicon wafers processed as a group (i.e., lot). Each integrated circuit is formed within a designated area of a wafer, and includes electronic devices electrically coupled by conductive traces called interconnect lines (i.e., interconnects). Interconnects are typically patterned from conductive layers formed on or above the surface of a silicon substrate. One or more conductive layers may be patterned to form one or more levels of interconnects vertically spaced from each other by one or more interlevel dielectric layers. Dielectric-spaced interconnect levels allow formations of densely patterned devices on relatively small surface areas. Interconnects on different levels are connected using contact structures formed in openings in the interlevel dielectric layers (i.e., vias). Following wafer fabrication, the individual integrated circuit dice are separated from the wafers, and each functional die is typically secured within a protective semiconductor device package.
The wafer fabrication process requires a high degree of precision, and it is important to identify and solve process problems as quickly as possible. Throughout the process a variety of tests and measurements are made to judge wafer and process quality. Allowing resolution down to submicometer levels, a scanning electron microscope (SEM) is useful in forming an image of integrated circuit structures. In a SEM, the source of illumination is an electron beam scanned over a surface. The incident (i.e., "primary") electrons cause bombarded materials on and just under the surface to eject "secondary" electrons. The energy levels (wavelengths) of ejected secondary electrons are detected and used to form an image of the surface, either on a viewing screen or as a photograph.
The SEM technique may be used to determine the dimensions of integrated circuit structures (e.g., thicknesses of interconnects) and to assess the integrity of contact structures. The integrated circuit may be scribed directly above the structure of interest and broken along the scribe line in order to reveal the structure of interest. The surface along the break may then be polished in a lapping process using successively finer grades of abrasive in preparation for SEM analysis.
When interconnects and contact structures are to be studied using the SEM technique, the integrated circuit may be immersed briefly (i.e., "dipped") in a mixture of hydrofluoric acid (HF) and water (H.sub.2 O) following polishing. The liquid HF--H.sub.2 O mixture removes silicon dioxide (SiO.sub.2) interlevel dielectric material surrounding the interconnects and contact structures, thereby forming a surface topography which highlights the interconnects and contact structures for subsequent SEM analysis.
FIG. 1 is a front elevation view of an apparatus 10 used for dipping an integrated circuit die 12 in a liquid HF--H.sub.2 O mixture 14. Integrated circuit die 12 includes a polished lower surface 16 being prepared for SEM analysis. Apparatus 10 includes a forceps 18 and a container 20. Forceps 18 are used to grip an upper portion of die 12 opposite lower surface 16 during the dipping operation in order to prevent damage to surface 16. Forceps 18 may be a compound lever forceps as shown in FIG. 1, and may be formed in one piece by injection molding. (See, for example, U.S. Pat. No. 3,636,954).
Container 20 is used to hold liquid HF--H.sub.2 O mixture 14. Integrated circuit die 12 is positioned between opposed jaws of forceps 18 such that lower surface 16 faces downward. Forceps 18 are positioned above container 20, and lowered until die 20 is 30 immersed in liquid HF--H.sub.2 O mixture 14. After a short period of time (e.g., a number of seconds), forceps 18 are raised such that die 12 is removed from liquid HF--H.sub.2 O mixture 14. Die 12 may then be rinsed in deionized water.
A problem arises when using apparatus 10. HF acid is highly corrosive, and the dipping procedure must be performed under a chemical vent hood. The dipping operation is performed manually, and a user must wear acid resistant protective equipment (i.e., gloves, apron, and safety glasses). Working under the vent hood forces the user to work with arms outstretched away from the body. The combined effects of gloved hands and outstretched arms makes it difficult to position forceps 18 such that die 12 remains immersed and lower surface 16 does not touch the bottom of container 20. Allowing lower surface 16 to touch the bottom of container 20 may damage the delicate interconnects or contact structures forming portions of lower surface 16 and no longer supported by surrounding interlevel dielectric material. When this happens, the damaged die 12 must be discarded. The steps of scribing, breaking along the scribe line to form surface 16, and polishing of the resultant surface 16 must be repeated upon another die 12.
It would thus be desirable to have an apparatus for immersing an object (e.g., an integrated circuit die) in a liquid (e.g., a liquid HF--H.sub.2 O mixture) wherein the apparatus includes a means of ensuring a lower surface of the object does not touch the bottom of the container holding the liquid. Such an apparatus would prevent damage to the lower surface of the object during the immersing.